1. FIELD OF THE INVENTION
The present invention relates to a process for producing a hydrated iron oxide. More particularly, it relates to an economical process for producing hydrated iron oxide particles used as a starting material for producing iron type acicular magnetic particles which are usually used for magnetic recording media.
2. DESCRIPTION OF THE PRIOR ART
Recently, hydrated iron oxide particles have been used in various fields and the demand for the hydrated iron oxide particles has been increased as pigments, a source of ferrite and a source for production of magnetic particles for a magnetic recording medium.
When the hydrated iron oxide particles are used as the source for production of the magnetic particles for a magnetic recording medium, the shape of the hydrated iron oxide particles directly relates to the shape of the magnetic particles. Therefore, it is necessary to produce hydrated iron oxide particles having a shape corresponding to a desired shape of the magnetic particles suitable for the magnetic recording medium.
It is well known that the important characteristics of magnetic particles for magnetic recording include a coercive force of magnetic particles, a dispersibility of magnetic particles in a magnetic paint for coating and an orientation of magnetic particles in a magnetic recording tape. As it is well known, these characteristics are important factors which highly affect to the characteristics of the magnetic recording tape. The shape of the magnetic particle directly relates to these characteristics of the magnetic particles. For example, when the coercive force is given by the shape anisotropy as the conventional magnetic recording medium, the coercive force is increased depending upon the improvement of the acicular property of the particles. The dispersibility is improved by decreasing branched or curved particles whereby the steric hindrance is decreased to improve the orientation and accordingly, the recording characteristics of the magnetic recording tape are improved. Therefore, it is important to obtain a magnetic particle having excellent acicular property with smaller branched shape in the preparation of the magnetic particles for a magnetic recording medium. In the industrial process, hydrated ferric oxide which is mainly goethite .alpha.-FeOOH which easily forms an acicular particle is firstly produced as a starting material and is converted into a magnetic metallic iron particle by heating, dehydrating and reducing it or is converted into Fe.sub.3 O.sub.4 particles by partially reducing it or is converted into .gamma.-Fe.sub.2 O.sub.3 particles by oxidizing the product.
In this process, the acicular property of the resulting particles depends on the shape of the starting goethite and accordingly, the shape of the goethite particles should be controlled. Therefore, it is important to provide an economical process for producing goethite particles having the desired shape in high efficiency in order to produce the magnetic particles for a magnetic recording medium.
In the conventional process for producing goethite which has been studied and employed as the industrial process, the ferrous hydroxide precipitate obtained by adding a base to a ferrous salt is oxidized in an acidic, neutral or alkaline solution. Various goethite particles having various shapes have been obtained by controlling the concentration of iron ion in the solution, pH of the solution, the temperature or the oxidation reaction velocity. However, in the conventional process, the goethite crystalline particles are precipitated depending upon mild oxidation of ferrous ion whereby it takes long time for the reaction and it is difficult to maintain the uniform oxidation reaction under a constant reaction condition. As a result, distribution of sizes of the resulting particles is broadened. In the conventional process, it is considered that hexagonal plate green rust is formed as the intermediate in the step of converting from ferrous hydroxide to goethite, and the dissolution of the green rust, the oxidation of ferrous ion and the precipitation of goethite as epitaxial reaction result on the surface of the particles. Therefore, goethite particles having a branched shape having an angle of about 120 degree are easily produced. This is a serious disadvantage.
As the other process for producing goethite, it has been known to use a ferric salt such as ferric chloride as the starting material. In this process, a ferric hydroxide precipitate is formed by adding a strong base to the ferric salt and the precipitate is converted into goethite by a hydrothermal reaction as the principle process. In this process, any oxidation is not required in comparison with the case using the ferrous salt as the starting material, and accordingly, the above-mentioned disadvantages can be eliminated. This process is remarkably advantageous in view of a production of the goethite particles having uniform distribution of sizes without branched shapes by utilization of the rapid goethite crystal growth reaction in the heating operation. However, this process has disadvantages in that it is necessary to form seeds of goethite before the hydrothermal reaction because this process utilizes the rapid crystal growth reaction under the condition of high temperature and high pressure. Even though the hydrothermal reaction is carried out without any ageing for forming seeds, only rough goethite particles or powdery hematite is produced. In order to produce suitable goethite particles, it is necessary to carry out the ageing treatment such as ageing at room temperature for about 24 hours. Thus, the conventional process for producing goethite from the ferric salt as the starting material also has the disadvantage of such requirement in the operation which should be improved.